Conventional electric motors have been thermally protected by use of PTC (Positive Temperature Coefficient) thermistors, i.e., temperature-sensitive resistance elements having positive resistance-temperature characteristics or presenting approximately uniform resistance at normal temperature but exponentially increased resistance above a particular temperature (the Curie point temperature). Specifically, among the components of the electric motor, a PTC thermistor is arranged at a point or therearound where a noticeable amount of heat is generated. The amplitude of the output signal from the PTC thermistor is monitored so as to issue an overheat alarm when the output signal exceeds a prescribed level and thereby turn off magnetic excitation of the electric motor.
The PTC thermistor output signal is preferably sampled as an electric signal such as voltage. Therefore, in order to convert the resistance presented by the PTC thermistor into voltage, an electric circuit including an appropriate fixed resistance and the PTC thermistor is prepared so as to measure the divided voltage across the PTC thermistor by applying a fixed direct current (DC) voltage to the electric circuit. In this way, it is possible to sample the voltage corresponding to the resistance, hence determine whether or not overheat happens, using the voltage as a parameter.
Particularly, in an electric motor including three-phase coils, a PTC thermistor is applied on the surface of the coil of each phase, and these thermistors are connected in series. This is because the PTC thermistors are connected with a single input and a single output so as to minimize the number of input/output terminals. FIG. 1 shows a conventional electric motor equipped with PTC thermistors. FIG. 2 shows a state of PTC thermistors mounted in the electric motor equipped with PTC thermistors. A core 1001 is provided with a U-phase coil 1021, V-phase coil 1022 and W-phase coil 1023, these three-phase coils having a U-phase PTC thermistor 1011, V-phase PTC thermistor 1012 and W-phase PTC thermistor 1013, respectively, on the surface thereof. These three PTC thermistors 1011 to 1013 are connected in series with a voltage-dividing resistor (not shown) having a fixed resistance by wire 1003, and applied with a prescribed voltage from a terminal 1004.
FIG. 3 shows a system configuration example of overheat detection for a conventional electric motor. When some coil is overheated, the resistance of PTC thermistors 1011 to 1013 sharply increases and voltage drop increases, hence the output voltage from the PTC thermistors increases. Accordingly, when the sum of the output voltage from the serially connected PTC thermistors 1011 to 1013 exceeds a prescribed voltage level that has been determined previously, overheat of, at least, one of the three coils is detected by an overheat determination unit 50, and the control unit of the electric motor 200 issues an overheat alarm.
There has also been another method of realizing similar overheat detection by connecting PTC thermistors in parallel instead of series to form a parallel circuit (for example, Japanese Patent Application Laid-open 2002-315383 (JP 2002-315383 A)). According to this method, substantially equivalent effect can be expected. However, if there exists some disconnection of wires in the parallel circuit, the PTC thermistor located in the disconnected part will not produce any output voltage.
Nevertheless, since the overheat detecting device receives output voltages from the PTC thermistors without a break, the overheat detecting device cannot detect presence/absence of disconnection. As a result, there occurs the risk that an overheat alarm cannot be issued despite the electric motor overheating, hence the overheat detection using a parallel circuit cannot be said to be a perfect measure.
When the electric motor continuously stops at a predetermined position under magnetic excitation, the maximum or almost maximum current flows through the coil of a particular phase, possibly overheating the coil of that phase. In this case, currents out of phase by predetermined angles flow through the coils of the other phases, hence the currents flowing therethrough are not so high as that through the coil of the phase in question, so that these coils will not reach an overheated state. Accordingly, if presence/absence of overheat in the electric motor is determined based on the total voltage across the multiple serially connected PTC thermistors, there occurs following problem. That is, when, despite an electric motor reaching an overheated state at a particular site, the sum of the output voltages of the multiple PTC thermistors has not reached the level over which an overheat alarm is issued, there occurs the problem that no overheat alarm can be output at the moment when a particular site alone is overheated.
Therefore, it is an object of the present invention to provide an overheat detecting device for an electric motor having an overheat determination function that can issue a quick overheat alarm even when a coil of a particular phase of the electric motor alone is overheated.